1. Field of the Invention
The present invention relates to an exposure apparatus, an output control method for an energy source and a laser device used in a field of production of semiconductor devices, liquid crystal display devices, etc.
2. Description of the Related Art
When producing semiconductor elements, liquid crystal display elements, image pickup elements (such as a CCD), thin film magnetic heads, etc. by using photo-lithography techniques, a projection exposure apparatus for exposing a wafer by projecting an image via a projection optical system of a pattern formed on a reticle as a mask on to a wafer (or a glass plate, etc.) coated with a photo resist, etc. has been used. As one basic function of the exposure apparatus, there is an exposure dose control function for controlling an integrated exposure amount (integrated exposure energy) so that it is within an appropriate range for respective points in each shot region of the wafer.
Recently, there have been demands for increased resolution of the pattern to be projected on the wafer. To increase the resolution, a method of shortening the wavelength of the exposure light has been used. As a light source for exposure light, pulse emission type laser light sources (pulse light sources), for example, excimer laser light sources such as KrF excimer lasers and ArF excimer lasers, metal vapor laser light sources, and YAG laser light sources, are currently known to emit light of short wavelength.
However, being different from continuously luminescent light sources, such as mercury lamps, there is a characteristic that the exposure energy of an emitted pulse becomes uneven between pulses in a pulse light source. Therefore, the unevenness of the pulse energy in exposure dose control must be considered when using the pulse light source. Generally, when using the pulse light source as a light source for exposure, a minimum pulse number, or so-called minimum exposure pulse number, to be irradiated on respective points on the wafer is regulated by considering the unevenness of the pulse energy in order to control the integrated exposure amount so that it is within a predetermined permissible range.
For exposure dose control in the case of applying the pulse light source to a projection exposure apparatus which is a collective exposure type, such as a stepper, so-called cutoff control using an integrator sensor for monitoring the pulse energy of exposure light for each pulse is commonly used. In the cutoff control, an integrated exposure amount supplied to a wafer is monitored during exposure based on information from the integrator sensor, and emission of the pulse from the pulse light source is stopped at the point where the integrated exposure amount supplied to the wafer exceeds a target integrated exposure amount. There is also a so-called every-pulse control by which the pulse energy is adjusted for every pulse emission in accordance with the information from the integrator sensor.
In the every-pulse control, the above minimum exposure pulse number can be reduced comparing with that in the cutoff control.
Controlling the energy of a pulse to be emitted next from the laser light source is important in order to improve control accuracy of the integrated exposure amount supplied to the wafer. Accordingly, measurement of the energy of every pulse by the integrator sensor provided inside the exposure apparatus and control of the energy of the next pulse based on measured energy information of at least one pulse previously emitted are performed.
In recent years, since the repeated emission frequency (repeated emission cycle) of laser light sources has become higher, as explained above, controlling the energy of the next pulse based on the measured energy information of the latest pulse previously emitted is difficult because sufficient time cannot be secured until an emission of the next pulse so the controlling accuracy of the energy of the next pulse declines in some cases.
For example, when outputting an exposure beam from a laser light source at a predetermined cycle, if detected energy information of a pulse output immediately before is used, calculation of the target energy of the pulse to be output next cannot be completed in time in some cases. Alternately, power charging of the laser light source for emitting the next pulse is not completed in time in some cases if the detected energy information of a pulse output immediately before is used. Furthermore, if the detected energy information of a pulse is output immediately before, an emission of the next pulse from the laser light source will be delayed in some cases. This is caused by an actual emission delay from the laser light source after sending an emission trigger from the exposure apparatus.